In a previous patent application of the Assignee herein, Sorin Biomedica S.p.A. (Italian patent application No. 67978 A/82) filed on Aug. 3, 1982 and corresponding U.S. Ser. No. 801,753, filed Nov. 25, 1986, there is disclosed a triode sputtering method and apparatus which enables the deposition of a biocompatible carbon coating on low temperature substrates (e.g. medical grade Dacron and Teflon) to produce carbon coated prostheses for implanation.
The carbon is described as turbostratic and is preferably deposited in thin layers of less than one micron.
Many fundamental requirements have to be fulfilled by a prosthesis coated with turbostratic carbon, such as, first of all, excellent adhesion of the coating to the prosthesis material (i.e., substrate), a complete and uniform coverage of the substrate, high resistance to wear and obviously, total compatibility of the coated substrate with the organic substances which come into contact with the prosthesis, in particular, blood. Complete and uniform coverage of the prosthesis is obtained by depositing a carbon film with a thickness of not less than about 0.1 micron and utilizing suitable systems for moving the substrate. Resistance to wear is obtained by the physical characteristics of turbostratic carbon and is optimized by means of suitable selection of parameters. Up to the present time, biological compatibility of carbon coatings has been obtained partially by coating with turbostratic carbon obtained by the triode sputtering method mentioned above. In fact, the turbostratic structure of carbon shows the highest biocompatibility that has been obtained up to the present. However, even higher levels of biocompatibility of the coating are still being sought. Research both on the physical characteristics of the coating which could possibly influence biocompatibility and the relevant parameters of the coating procedure and also the reciprocal relationship between parameters have been considered by the applicants herein. In vivo evaluation has been performed in animal tests which have enabled the present inventors to evaluate the biocompatibility of the materials which have been obtained. It has been found that the factor which has the greatest influence on the biocompatibility of the coating is the density of the carbon coating.
It is known in the art to make carbon coated prostheses having a coated density in the range of about 1.5 to 2.0 grams per cubic centimeter. However, the level of biocompatibility of such carbon coated devices has not satisfactorily fulfilled the present requirement of providing prosthetic devices with a high degree of biocompatibility.
Applicants' efforts to increase the density of the carbon coating have been directed to all types of synthetic materials normally in use for prostheses to be implanted into the human body and, in particular, the following materials have been coated: polyurethanes, silicon elastomers, polyesters, polyethylene, polytetrafluoroethylene and polyacetal resins in all possible shapes, in the form of both layers and fibers, yarns and fabric and in more complex shapes. These materials have been coated with turbostratic carbon obtained by the cathode sputtering method described in the previously mentioned Italian patent application No. 67978 A/82 and (U.S. Ser. No. 801,753) incorporated herein by reference) and the parameters of the process (tensions, currents, temperatures, pressures) have been set up according to reciprocal relationships which have been varied in order to obtain the density which was found to yield the desired results. For all such materials the biocompatibility of the coating has been found highly improved with respect to that of coatings obtained using prior methods and lower densities.
Referring to the first requirement mentioned above which is the adhesion of the coating to the substrate, it has been observed that by increasing the density of the carbon coating to improve biocompatibility, the coating is also highly adherent to the substrate.
It is therefore an object of the present invention to provide a method of coating a prosthesis with a thin film of carbon which exhibits an excellent degree of biocompatibility and is firmly adherent to the substrate material.
It is another object of the invention to produce a carbon coating which has a density of at least 2.1 g/cm.sup.3 and, preferably at least 2.2 g/cm.sup.3.